The gender pay gap for preschool and early childhood education workers is more than twice the national rate, despite women being 89.3% of all employees in the sector.

Data released by the Workplace Gender Equality Agency shows female full-time employees in the preschool sector were paid on average 31.9% less than men in the same industry, compared with a national gender pay gap of 15.3%.

The information was released for equal pay day on Monday and comes as thousands of early childhood educators and preschool workers prepare to take part in national stop-work action to protest against unfair rates of pay, which they say are 30% less than rates paid in comparable male-dominated industries.

"There's no greater indication of the lack of importance that we attach to things that have traditionally been seen as women's work than early childhood education," Helen Gibbons, the assistant secretary of United Voice, the union that represents childcare workers, told Guardian Australia. "When it was provided inside the home it was undervalued and it continues to be undervalued outside the home as well."

Gibbons said some childcare workers were paid as little as $21 an hour, about half the national average wage, despite being required to have studied for a minimum of 18 months for qualifications ranging from a certificate three in childcare to a four-year education degree.

"It's exactly the same degree as if you were working in a school. It still takes four years to do, it still costs you a significant amount of Hecs debt, but on average you are earning 30% less than a primary school teacher," Gibbons said. "How is that fair? Two-year-olds aren't any less important than six or seven-year-olds."

About 3,000 childcare workers are expected to walk off the job at 3.20pm on Thursday to mark the time at which, according to the global gender equality clock, women start to work for free.

The clock is based on the average pay gap in the US, where women earn on average 79% less than men.

The pay gap is slightly smaller in Australia, where women earn on average 15.3% less than men, according to data released by Workforce Gender Equality Agency. It found that the average weekly wage of a woman working full-time in Australia in 2016 was $1,387, about $251 less than men, who earned $1,638.

United Voice said the average pay for childcare workers, 97% of whom are women, was $809 a week for those with a certificate three qualification; $911 for those with a diploma; and $1,009 for those with a four-year early childhood education degree.

"When it comes down to it, when they want to have their own children and settle down and afford a mortgage, they can't do it on their wage," Gibbons said. "Ironically, so many of them leave at the stage that they want to have children, because they can't afford the childcare."

A wage case petitioning for a 30% pay rise, based on average wages paid in the metalwork industry, chosen because the male-dominated industry has similar tiered qualification requirements to early childhood education, has been before the Fair Work Commission since 2013.

It has yet to reach the hearing stage. Gibbons said workers were calling on the federal government to increase funding to childcare to allow providers to increase wages without passing the cost on to parents, who already pay high rates.

The walk-out follows a similar action in March, where 1,000 childcare workers left early. An estimated 10,000 families will be affected by Thursday's action.